1. Field of the Invention
The present invention relates to a small-sized thermal printer mounted on a desk-top type electronic apparatus that is installed in a shop or on various portable information devices, and more particularly to a thermal printer having a structure in which, even if a platen roller is of a detachable type, there is no positional displacement between a thermal head and a platen roller during the printing operation.
2. Description of the Related Art
A conventional example of a general small-sized thermal printer provided with a line-type thermal head will now be described with reference to FIGS. 7 to 10.
The thermal printer P2 shown in FIG. 7 is provided with a platen roller 102 pivoted rotatably to a frame 101, a head support member 103 provided with a thermal head (not shown in FIG. 7) facing the platen roller 102 and a platen spring 104 for giving a biasing force for pressing this head support member 103 against the platen roller 102.
Also, the thermal printer P2 of FIG. 7 is provided with a head-up lever 105 or the like having a cam for moving the above-described head support member 103 apart from the platen roller 102 during the maintenance operations for the paper clog due to paper jam or for replacement of recording paper. The head-up mechanism composed of this head-up lever 105 is a mechanism provided for the purpose of making it possible to manually separate the platen roller 102 and the head support member (thermal head) 103 away from each other by a predetermined distance (for example, at an interval of several millimeters) in order to enable, for example, replacement operation of a thermal sensitive type recording paper or operations for removing paper clog due to paper jam that occurs almost unavoidably.
In the thermal printer P2 shown in FIG. 7, since the platen roller 102 is of a fixed type such that the roller cannot be detached, there is an advantage that the positional precision between the thermal head and the platen roller 102 is high.
Also, a driven gear (not shown in the drawing) to be engaged with a gear train provided with a motor M as a drive source is provided on one side (on the right end side in FIG. 7) of the platen roller 102. Since the roller 102 is of the fixed type as described above, the force applied to the driven gear when the platen roller 102 is rotationally driven may be absorbed by a shaft fastening portion of the platen roller 102. Thus, there is no problem that the platen roller 102 is lifted during the driving operation to cause positional displacement relative to the thermal head.
Although the thermal printer P2 has the advantages as described above, the thermal printer has a disadvantage that the operationability in inserting the recording paper is poor. Namely, when the recording paper is to be inserted as described above, the head-up lever 105 is operated to separate the thermal head and the platen roller 102 away from each other by several millimeters and the recording paper is inserted into the gap thus produced. However, insertion of the recording paper is often difficult when the recording paper is curled or its end is folded. Accordingly, there is a demand to improve its structure.
Therefore, a thermal printer P3 that has a structure such that a platen roller 202 may be detached as shown in FIGS. 8 and 9 has been developed.
This thermal printer P3 is composed of a frame 202 having a pair of side wall portions 201a and 201b arranged to face each other through a predetermined interval in a paper width direction, a head support member 204 for holding a thermal head 203, a platen roller attaching/detaching mechanism Axe2x80x2 for detachably and rotatably bearing the platen roller 210 by means of the above-described pair of side wall portions 201a and 201b, a pressing mechanism Bxe2x80x2 for bringing the surface of the thermal head 203 into contact with the circumferential surface of the above-described platen roller 210 with predetermined pressing force, a gear transmission mechanism G1 for transmitting the rotational driving force of the motor M to a driven gear G2 fixed at one end (at the right end in the drawing) of the above-described platen roller 210, and so on. In the structural example shown in FIGS. 8 and 9, the pressing mechanism Bxe2x80x2 is formed integrally with the platen roller attaching/detaching mechanism Axe2x80x2 and they operate as an integral mechanism as a whole.
As shown in FIG. 9, the platen roller attaching/detaching mechanism (pressing mechanism Bxe2x80x2) Axe2x80x2 is provided with a swing member 300 having a U-shape as a whole and which is slightly wider than the thermal head 203 (head support member 204). The swing member 300 is provided with a pair of hook-like latch portions 301 to be engaged with both shaft ends (210a and 210b) of the platen roller so as to draw the platen roller 210 toward surface of the thermal head 203, a pair of arm portions 302 for guiding the latch portions 301 to the rear side of the head support member 204 along both sides of the thermal head 203 (head support member 204), and an elastic member support portion 303 provided at its both ends with arm portions 302 for supporting springs S10, S10 as the spring members and arranged in the width direction in the rear of the head support member 204. The swing member 300 and the head support member 204 are swingably borne through a support shaft 400 (see FIG. 10) provided between the side wall portions 201a and 201b. 
Furthermore, a release lever 500 serving as an engagement release means for swinging the swing member 300 itself forwardly so as to release the engagement between the latch portions 301 and the both shaft ends 210a and 210b of the platen roller 210 is provided in the rear of the swing member 300 (see FIG. 8).
The thermal printer P3 having such a structure has such an advantage that, when giving a new supply of the recording paper or the paper clog occurred, the release lever 500 is operated to activate the platen roller attaching/detaching mechanism Axe2x80x2 to remove the platen roller 210 from the frame 202 to thereby make it possible to perform the removal of the paper causing the paper clog, giving a new supply of the recording paper, or the like.
Nevertheless, the thermal printer P3 shown in FIGS. 8 and 9 has the above-described advantage, however, the printer suffers from such a disadvantage that the positional displacement between the thermal head 203 and the platen roller 210 can easily occur due to such arrangement that the platen roller 210 is of the detachable type.
Namely, in this thermal printer P3, various forces (F1, F2, and F3) as shown in FIG. 10 are applied to the platen roller 210 and the composite force (F4) thereof works in a direction to lift the platen roller 210. Accordingly, the positions of the thermal head 203 and the platen roller 210 are displaced from each other upon the execution of the printing operation.
More specifically, under the state where the shaft end 210 of the platen roller 210 at which the driver gear G2 is provided, is being held to the latch portion 301, the pressing force F1 to be applied via the thermal head 203 (head support member 204), the pressing force F2 to be applied via the arm portion 302 and the latch portion 301 (i.e., the force for drawing the platen roller 210 to the side of the thermal head 203) and the pressing force F3 to be applied via the driven gear G2 due to the drive of the gear transmission mechanism G1 upon the execution of the printing operation, which works in counterclockwise direction in the drawing, are applied to that shaft end 210. In particular, a part of the rotational force of F3 is converted into the force that works in a direction for moving the shaft end 210b apart from the latch portion 301, due to frictional force between the latch portion 301 and the circumferential surface of the shaft end 210b. Then, the force F4 which works in a direction for lifting the driven gear G2 side of the platen roller 210 away from the latch portion 301 is generated as the composite force of these forces F1, F2 and F3. As a result, there is a problem that the positional relationship between the thermal head 203 and the platen roller 210 is deviated from the right positional relationship.
As an approach to prevent such a lifting phenomenon of the platen roller 210, the spring pressure of the springs S10, S10 as the elastic members may be increased (for example, by using springs having higher elasticity instead or increasing the compression amount of the springs) to strengthen the pressing forces Fl and F2 to be applied to the roller 210, to thereby practically suppress the effect of the composite force F4. However, if the spring force of the springs S10, S10 is excessively increased, the frictional force between the circumferential surface of the platen roller 210 and the thermal head 203 and the frictional force between the latch portion 301 and the circumferential surface of the shaft end 210b are increased to increase the load on the motor M for rotating the platen roller 210. As a result, a new problem occurs such that the rpm of the platen roller 210 (delivery speed of the recording paper) is reduced, resulting in decreased printing speed.
In order to overcome the above-noted problems, an object of the present invention is to provide a thermal printer that may prevent occurrence of the lifting phenomenon of the platen roller without decreasing the printing speed.
In order to attain this object, according to the present invention, there is provided a thermal printer comprising: a frame (1) having a pair of side wall portions (2a and 2b) arranged to face each other at a predetermined interval in a paper width direction; a head support member (4) for holding a thermal head (3); a platen roller attaching/detaching mechanism (A) for detachably and rotatably bearing a platen roller (10) by the above-described pair of side wall portions; a pressing mechanism (B, coil springs S1 and S2) for bringing a surface of the above-described thermal head into contact with a circumferential surface of the above-described platen roller with predetermined pressing force; and a gear transmission mechanism (G1) for transmitting a rotational driving force of a motor (M) to a driven gear (G2) fixed at one end of the above-described platen roller, wherein a pressing force applied on a shaft end on the driven gear side of the above-described platen roller by the above-described pressing mechanism is made to be within a range of 1.3 to 3.0 times of the pressing force applied on the other shaft end.
Thus, it is possible to prevent the lifting phenomenon of the platen roller upon the execution of the printing operation and to thus prevent displacement between the platen roller and the thermal head. In addition, pressing force applied on a shaft end on the driven gear side of the platen roller by the above-described pressing mechanism is made to be within a range of 1.3 to 3.0 times of pressing force applied on the other shaft end, whereby it is possible to suppress as much as possible the increase of the load on the motor caused by the frictional force between the circumferential surface of the platen roller and the thermal head and the frictional force between the latch portion and the circumferential surface of the shaft end to thereby avoid reduction in the printing speed.
Incidentally, it is preferable that the average of the total pressure to be applied to the circumferential surface of the platen roller by the above-described pressing mechanism be in the range of 15 to 35 g/mm. Thus, it is possible to suppress the increase of the load on the motor more effectively and thus to avoid the reduction in printing speed more reliably.
Also, the above-described pressing mechanism is formed integrally with the above-described platen roller attaching/detaching mechanism, the above-described platen roller attaching/detaching mechanism is provided with a swing member having a U-shape as a whole and which is slightly wider than the above-described thermal head, the swing member is provided with a pair of latch portions to be engaged with both shaft ends of the above-described platen roller so as to draw the platen roller toward the surface of the above-described thermal head, a pair of arm portions for guiding the above-described latch portions along both sides of the above-described thermal head to the rear side of the above-described thermal head, and an elastic member support portion provided on its both sides with the above-described arm portions and arranged in a width direction in the rear of the above-described head support member for supporting elastic members, the above-described swing member and the above-described head support member are swingably supported through a support shaft provided between the above-described side wall portions, an engagement release means for moving the swing member to thereby release the engagement between the above-described latch portions and the above-described platen roller is provided in the above-described swing member, and one or two or more elastic members adapted such that pressing force applied thereby on a shaft end on the driven gear side of the above-described platen roller is 1.3 to 3.0 times larger than that applied on the other shaft end may be held between the elastic member support portion of the above-described swing member and the above-described head support member. Thus, it is possible to prevent the lifting phenomenon of the platen roller upon the execution of the printing operation to thereby prevent displacement between the platen roller and the thermal head.
Also, two or more above-described elastic members may be used and each of the above-described elastic members is arranged and positioned such that a barycenter of pressure applied on the above-described platen roller is deflected toward the shaft end on the driven gear side of the above-described platen roller by 7 to 25% whereby the pressing force applied on the shaft end on the driven gear side of the above-described platen roller may be 1.3 to 3.0 times larger than that applied on the other shaft end. Thus, simply by suitable changing the arrangement positions of the elastic members, it is possible to achieve prevention of the lifting phenomenon of the platen roller upon the execution of the printing operation.
Also, in the case where two or more above-described elastic members are used, elastic members having the same elasticity may be used. In that case, the compression amounts of the respective members are varied so that the pressing force applied on the shaft end on the driven gear side of the above-described platen roller may be 1.3 to 3.0 times larger than that applied on the other shaft end. Thus, it is possible to use elastic members of identical specifications. Therefore, it is possible to achieve prevention of the lifting phenomenon of the platen roller upon the execution of the printing operation without increasing the manufacture cost.
Also, in the case where elastic members having different elasticity are used as the above-described elastic members, each elastic member is arranged at the equal distance from the center, whereby due to the difference in the elasticity, the pressing force applied on the shaft end on the driven gear side of the above-described platen roller may be 1.3 to 3.0 times larger than that applied on the other shaft end. Thus, by suitably arranging two or more elastic members having different elasticity, it is possible to achieve prevention of the lifting phenomenon of the platen roller upon the execution of the printing operation.
Incidentally, the above-described elastic member may be made of a spring in general, rubber, synthetic resin or the like.